Radio aid to navigation



Dec. 29, 1953 J. LYMAN ET AL RADIO AID TO NAVIGATION Filed Deo. 1, 1949 3 Sheets-Sheet l lm u J L/ Z5 /5 Q3 lNvENToRS A'TTORNEY Dec., 29, 1953 J. L YMAN T AL RADIO AID TO NAVIGATION Filed Dec. l, 1949 3 Sheets-Sheet 2 RECEIVER ATTORNEY Dec. 29, 1953 J. I YMAN ET A1. 2,664,560

RADIO AID TO NAVIGATION GEORGE 5. L TCA/FORD Patented Dec. 29, 1953 NH'E Slid'ii PAYER? @FFME RADIO All) T NAVIGATION Joseph Lyman. Huntington, and George B. Litchforcl, Cold Spring Harbor, N. Y., assignors to The Sperry Corporation, a corporation of Dela- 13 Claims.

This invention relates to radio aids to the navigation of aircraft, and more particularly to improveznents in instrument landing or ground approach systems. wherein radio means are used to guide an aircraft or direct the pilot thereof so as to a predetermined path. such as a glide path.

Prior art radio glide path systems have generally been either of the constant intensity type or of the eouisignal type. In the constant intensity system, a single stationary beam is directed over the landing strip in such manner that one of its lower contours of Uniform eld strength conforms as nearly as possibleV with the desired `glide path. The airplane is flown so as to keep the received signal strength constant at the proper level, and thus the pattern contour is followed to or near the ground.

The equisignal type glide path system uses two overlapping beams, one above the other (i. e. at a higher angle) and distinctively modulated. The center of the overlapping region is made to coincide with the desired glide path. The aircraft is flown so as to maintain the intensities of the distinctively moiulated signals eoual.y

The constant intensity glide path system is open to the obiection that the proper contour must be located by independent means, such as an altimeter in the aircraft used in coniunction with one or more marker bea-sons at check points along the approach path. Moreover, the path is not straight throughout any substantial portion of its length, hut curves from a relatively steep slope where the aircraft enters the contour to a nearly flat portion near the gro'rnd.

The equisignal glide path system will denne a single sti ight path without requiring any external calibration, and within the zone of overlapx of the tif-o beams, the amount of deviation of the aircraft above or below the eguisisnal plane is indicated oy the difference in amplitude of the two received signals. However, two antennas are usually reouire'l. and they may be spaced apart vertically by a considerable distance. Although they are energized from a common trans-rn'. ter, the antennas must be supplied through separate feed lines and modulators, constituting two substantially independent channels. Any variation in the transmission through one channel with respect to that through the other will change the angular elevation of the equisignal The use of separated radiator-s introduces other diiiiculties in addition to the possibility of variation the position of the equisienal plane.

The apparent origin of the radiation becomes ine` definite as the lower end ofthe glide path is approached; the equisignal surface is not ideally plane, but may be undesirably curved, andcan be so distorted as to produce a roller coaster effect.

Since the antenna system must be offset laterally from the landing strip so as not to obstruct it, the horizontal radiation pattern must be wide enough to cover the entire glide path, and the vertical patterns should be uniform throughout the angle of horizontal coverage. This condition is substantially impossible of fulllment with vertically spaced antennas; in practice, a compromise is eiected by making the horizontal patterns of the upper and lower antennas diiferent, in such manner as to balance one type of distortion by another'. It will be apparent that this sort of expedient tends to increase the diioulty of adjusting the system for proper performance; and to aggravate the eects of any random variations in transmission through the two feed channels.

As mentioned above, the equisignal system provides quantitative information about Vertical deviation from the glide path within the overlappingzone of the two beams. Outside this zone, the indication is merly up or down, and the extent of the deviation is not shown. In order to provide for the different characteristics of different aircraft, it is desirable to allow a choice of glide path angle, making it possible to fly a path somewhat steeper or less steep than the equisignal plane. Preferably the selection of glide angle should be made in the aircraft. This can be done by operating with the indicator off-center, by adjusting the gains differentially for the different modulation signals, or by applyin; a bias to the indicator. The adjustment may be made as a function of signal strength or distance from the touchdown point to provide are, or decrease of slope as the ground is approached, for example.

Since an aircraft above or below the overlap zone of thetwo beams will receive no infor-ination as `to how farit is above or below the equisignal plane, the choice of glide angles is limited to this Zone. The angular width of the overlap zone must be kept fairly small to provide a reasonably large response to small deviations of the aircraft from the glide path. Thus a compromise isnecessary between the contradictory requirements of large angular coverage in elevation and accuracy of operation.

One of the principal objects ci the present nvention is to provide radio beam systems for the :uidance of aircraft wherein large variations in he transmission characteristics of the feed sys- `em or in the amplitude of any of the radiated ,ignals can have no substantial effect upon the tccuracy of operation of the system as a whole.

Another object of the invention is to provide systems of the described type using a single raiiator structure, thus avoiding the various dini- :ulties which result from the use of two verti- :ally spaced radiators.

A further important object is to provide a glide path system or the like which has no equisignal plane or similar principal axis, but provides unambiguous quantitative elevation angle information throughout a wide angular range.

Still another object is to provide a system of the described type capable of providing uniform coverage throughout a Wide angle of azimuth.

Another object is to provide a glide path system in which the angle of the glide path and also its shape, curvature or flare can be controlled on board the aircraft.

It is also an object of this invention to provide radio guidance systems including improved. height nnder means for determining the height or altitude of an aircraft above a reference plane such as that of a landing field, irrespective both of the barometric altitude and the terrain clearance or absolute altitude.

It is also an object of the invention to provide a combined radiator and modulator structure for producing a directive radio beam of the type peculiar to the presently contemplated guidance systems.

The invention will be described with reference to the accompanying drawings, wherein:

Fig. 1 is a schematic block diagram cf a radio glide path system embodying the present invention,

Fig. 2 is a perspective view, partly broken away to show internal details of an antenna structure suitable for the system of Fig. 1,

Fig. 3 is a plan section of the structure of Fig. 2,

Figs. e, 5, 6 and 7 show the radiation pattern in a vertical plane of the antenna of Figs. 1-3 at various instants during the modulation cycle,

Fig. 8 is a diagram similar to that of Fig. 4 but in addition showing the phase relationship between a reference phase modulation and the modulation caused by field pattern variation at Various elevation angles,

Fig. 9 is a schematic block diagram of the airborne portion of a system for indicating the elevation angle of an aircraft with respect to a reference plane,

. Fig. 10 is a schematic block diagram of a modification of the system of Fig. 1 to provide a curved or flared glide path,

Fig. i1 is a schematic block diagram of a height finder system embodying the invention and incorporating some of the elements of a glide path system like that of Fig. 1, and

Fig. 12 illustrates a mechanical computer device suitable for use in the system of Fig. 11.

The glide path system shown in Fig. 1 includes a ground station located in the vicinity of an airport runway or landing strip, and an aircraft station carried aboard each craft which is to use the system. A transmitter l and a special directive antena 3 are provided at the ground station. Preferably the transmitter is designed to operate at a frequency of the order of 5000 megacycles per second. The antenna 3 is arranged to provide a beam which is space-modulated in a phase which varies with elevation angle, as will scribed.

Figs. 2 and 3 illustrate a type of antenna which is at present preferred for use as the element 3 of Fig. 1. A small directive horn e is connected to and may be supported on the end of the outer conductor of a coaxial transmission line The inner conductor of the line 'a'. extends into the horn and terminates therein as a radiator element 9. The axis of the line l and the radiator 9 is substantially horizontal.

The horn 5 is surrounded by a drum-shaped structure comprising parallel conductive discs or plates Il and i3, connected together at their peripheries by bars l5 of insulating material. An aperture Il is provided in the disc i i for the line 1, and the drum assembly is supported at the end of a shaft is for rotation about the axis of the radiator 8.

A plurality of phase front modifying elements 2i are provided around the edge of the drum structure at intervals equal to or somewhat greater than the height of the mouth of the horn 5. The elements 2t comprise conductive stubs arranged in pairs extending inwardly toward each other from the discs l! and I3.

A second horn 23 is arranged substantially as an extension of the horn outside the drum assembly. A lens system, including a dielectric lens 25 at the mouth oi the horn 23 and a cellular metal lens 2e (Fig. 1) in front of the horn, may be provided for producing a sharper beam than that which would result from the use of the horns 5 and 23 alone.

Returning to Fig. 1, the transmitter i is connected to the antenna 3 by way of the line l. The shaft le extends to and is driven by a motor 2l at a nominally constant speed of, for example, 30 revolutions per second. A reference phase generator 20 comprising a small alternator is also driven by shaft I9 in synchronism with the drum assembly of the antenna s. The frequency of the generator 3 is the same as that of the passage of the phase front modifying elements 2i across the mouth of the horn 5. rihus, assuming that there are ten of the elements 2i, the generator 29 is arranged to operate at 300 cycles per second.

A modulator 3i is supplied with the output of the generator 20 and also with the output of a subcarrier generator or oscillator 33, which may operate at a frequency of say 10,000 cycles per second. The modulator 3i provides an output comprising a 10,000 cycle subcarrier which is modulated by the 300 cycle output of the reference phase generator. This output is applied to the modulation input terminals of the transmitter I to modulate the high frequency carrier. Either of the carrier and subcarrier modulations may be in amplitude or in frequency.

The aircraft station includes a receiver 35, designed to respond to radiation from the ground station and to reproduce the modulation provided by the modulator 3i. The output of the receiver 35 goes to a low frequency filter 37, designed to pass 300 cycles per second, and also to a subcarrier iilter 39 which passes 10,000 cycles per second. The subcarrler signal is app-lied to a dernodulator i to reproduce the 300 cycle reference phase signal originating at the generator 2S.

A phase detector` di, is provided with one of its input circuits connected to the 300 cycle filter 37 and its other input circuit connected, by way of an adjustable phase shifter 35, to the demodulator 4l. The output of the phase detector A3 is be deinzazrnuthssay forty-ive degrees- Qwing, to the..

presentent oneof theiphase front modifying elee menisll, in the spaoebetween thehorus. 5 and 4 3, the .beam is not. uniform in intensity; but

varies moreVv or vless smoothly rom aiminimum,

as'indioated at .t in Fig; .4, toa maximum as at5.3;.

Theielevation angle .dat which thev maximum or--peaii 53 .appears at any particular instant. depends. upon the. position o-ffthe phase front modifying element il between the horns and' 23; and thus upon the angular positionn o'fzthe.

rotatable drumassemhly. As the drum is rotated by the motor 2; the Variations in intensity ol the beam move likewsen elevation; Fig.. i'ishows the shape of the `hea-n1 when the-drum has rotated' tosmove 'the phasefront modifying element down ward from the position corresponding t'oFig. 5'

loyaY distance of about one-fourth the-width of the mouth of the horn. It is seen that the maximum 53' has moved downward through a certain ang-le, andthe minimum 5I hasfmoved l partly off thel bottom ot thevpattern-,while a new minimum 5| f has Ybegun to-appear-at the top.

VIrrFig.- 6, which oorrespondsto furtherlrotae torof the drum Athrough an angle-equal to that between theipositions*ofr 4 ando; thema-Ximuln 53 ;has: moved totheliottorr of the pattern.

Fig; 7, 1 representing'a still' `later positionofthe drum, shows-themaximum 53 partlyoil the bottom ofthe pattern, while a new: maximum 53 is beginning to` enter the top. As the drum. oontinues toA rotate, the. eye-le represented by Figs..

4r? is repeatedpelodioally, with 30G maxima and 300 minimaper second passilgthroueh any given line oi elevatimzlL The. intensi-ty o the-radiation arriving atthe.

airborne receiver sgvariesoyolically, at the. frequency 01.2300 cycles persecond, and this. Varia.- tion is .exactly the'saine as itthe Signal Were modulated at that'freouency at the.. transmitter..

The instantaneous: amplitude at any particular 5?;-

instant'is a iunotion of elevation angle, as shown by Fig-S. elf-and thus the phase of the pattern- Variation modulation depends uponthe elevation angle of the receiver 35, above the horizontal planefthrough the transmitter.

Theinstantaneous magnitude of the output of the reference phase generator 28 on the .other hand is ind op-ancient of: the: oosill of. the receiver, and isa funotion only of angular position ofthe shaftlSf.' Thusjthe :phase dieleence hetweenf the reproduced' 300; cycle.' referenoe. phase Signal, v appearing at the output: of*` he` demodulator si, and the 300 cycle pattern-variad tion modulation signal coming out oi thegltel 3l, isdirectly related to. the elevation angloool the airho1-nereceiver- A .Referrineto'e 8, a radiation pattern like that ofFig'; 4: is shown1 withosoillograms on various., elevationlines illustrating` thephase relationships between-the'two 390 eyolesignals at .those elena.:`

tiene. :.-llherdotted lineazenreserit-the referenoe variation.. between the two signals will depend upon :the oyerall .width @tithe beam and the number o phase .ntniodifying elements 2.! in the dnimiasseiubiy.. Preferably thesefactors are S0 Lelatedthatzthertotal phase variation between the tonandhottoor oit-he hea-io issomewhat. less thaniiedeareos..

Lilie :ulzlase shifter @iii is adjusted. by means 49 aooordioatoithe.;elevation. .allele 0i the desired slhierpath.; so; that. as long .as the airborne equinmentison .the .seleoted elevation line, the phase relationshiehetween the two 30o Cycle inputsto thenhasedeteotor a3 such to makethe out.,- outtthe; dicatorr il: zero. Thereafter, asthe ailoraftgdevglates ove or below the glide path, the extent .end dreetienof the departure is indicated.

It Vshould .be-noted that the glide path angle is selected. onfthe aircraft itself, and that such selectiondoesnot involve any adjustment oi the groundstation equipment and does not affect the Signals.' reoeivedby any other aircraft. More oven'the'rangeof glide path angles which can beseleotedis limited only by the vertical coverageof the-beam, and is not confined to a .relatively small .Crossover region between two beams. There is noprinoipal orpreerred elevational linel andthe accuracy of; thesystem is the .same at all eleuationangles .withinthe beam.

, .Sinoetherefis only one beam. involve he in: dcations arer independent oi, ahyvar. tions in the .tra-osmttenoutput.; Only one radiator and feed system is required and the radiation comes fromzasinele:apparent.origin. Fie. 9; Shows a modification of .the airborne portion of the sys tem of` Fig.- 1, for providinga direct indication of the-angle of elevation instead of indicating deviatolfrfronl.A a selected angle. The only diierenoe between :this systeme-nd that of Fig. l is that the outputzof .thephase ,detector d3 goes to. a servo or ffolloweup. loop comprising an. amplifier 55 and areversble motor 5.1.. The motor 57 is arranged to drive; the phase shifter t5, and its angular position is indicated in terms of elevation angle by a dial and pointer 59. In operation, the output of the phasedeteotor a3 Vcontrols the motor 5l to adjust .the phase shifter i5 so that the phase deteotor output goesto zero. This positions the indicator ilaocording to the amount of phase shift; and thus laccording to the elevation angle.

Theydeviation indicating function. of Fig. l may beretained Fig. 9 by providing a double throwswitoh Slgfor applyingthe phase detector output :to .eithertheemphase-e5 or. the zeroeeni ter. .metersl, and including. a disensaeeable clutch inltiheeonneotion between the moto;` 51:- anclgthefphase shifter 5. When the switch ftlsith'iewn t0 the-right and the ciu-tch e3 is disengaged,v the-servo loop is disabled and the operation of; File,n 9hecoimas the same as that of Fig'.- v1;.

@ne ,obieetion :which has been made about straight line glide path sys-tems; is that the aircraft; reaehestherground in a diving attitude, whiohgmay result in e. bumpy landing. To prelent this-itisdesirableto curve or dare the lower end of the glide path in such manner that the steepness is gradually reduced and the aircraft approaches contact in a substantially level attitude. Another characteristic of glide path systems in general which sometimes causes trouble is the increasing response to minor deviations from the path as, the ground station is approached. This effect may be reduced by the procedure known as course softening which involves the automatic reduction of the deflection sensitivity of the deviation indicator as the distance to the ground station decreases.

Fig. l shows a modification to the present system, incorporating means for producing flare and course softening. The glide path transmitter system, corresponding to the ground station of F'ig. 1, is represented by the block c5. The airborne station includes the receiver 35,. phase shifter 45, phase detector 63 and deviation indicator All, substantially as in Fig. 1. In addition to the basic glide path system of Fig. 1, there is provided a distance measuring system or DME (distance measuring equipment) of known type which comprises a transmitter-receiver combination l at the aircraft station and a receivertransmitter combination @Si at the ground station. Radio transmissions are originated at the device El, received by means es at the grornd station and relayed back to the aircraft station where the initially transmitted signals are compared with the finally received signals to produce an output corresponding to the round trip radiation travel time and therefore to the distance. The output of the DME unit el is applied to a servo or follow-up system ll, whose output shaft 'i3 is angularly positioned according to the distance of the aircraft station from the groundV station. A cam 75 cut according to the desired relationship between elevation angle and distance for terminal curvature of the glide path isdriven by the shaft 'l and operates a cam follower to control the phase shifter d5.

An adjustable attenuator Tl is provided in the connection between the phase detector 43 and the deviation indicator The attenuator 'l'l is .f

mechanically coupled to the DME servo shaft 'I3 as indicated by the dash line I9. The connection it may include a suitable cam arrangement, or alternatively the attenuator 'l'l may be designed to have an attenuation vs. rotation characteristic which is tapered according to the desired Variation of deviation response as a function of distance for course softening.

In operation of the system of Fig. the phase shifter 135 is controlled by way of the cam 'l5 as a function of the distance, providing a glide path which is curved according to the cam design. It should be noted that each aircraft or each type of aircraft may have a cam which is designed according to its own particular flight characteristics, and that the glide path selected for one aircraft does not place any limitation on the glide path which may be selected for any other aircraft. The attenuator l?, also driven from the DME shaft l?, simply reduces the deflection sensitivity of the indicator 4l as the ground station is approached. This prevents eXtreme deflections of the indicator 47 in response to minor deviations of the craft from the desired glide path, enabling the pilot to keep the aircraft reasonably close to the desired path without requiring unduly accurate control.

In ground approach and other aircraft operations, particularly in the vicinity of landing fields, it is frequently desirable -to secure accurate altitude information. Altimeters of the terrain clearance indication type measure the altitude above the surface directly under the craft. Since many airports are surrounded by irregular terrain, this measurement does not tell the height over the landing field itself. According to the present invention, the height of an aircraft above the reference plane is determine-d independently of the terrain by triangulation based on measurements of the slant distance and elevation angle of the aircraft from the ground station.

Fig. 11 shows the airborne portion of a system for providing height indication. The ground station portion of the system may be identical with that of Fig. 10. 'Ihe elevati-on responsive portion of the system, including the receiver 35, phase detector 43, phase shifter 45, motor amplifier 55 and motor 51 is arranged substantiallyzs the corresponding elements are arranged in Fig. 9. The motor 51 is also coupled to a sine function generator Bl, which provides an output either in the form of an electrical quantity such as voltage, or a mechanical quantity such as displacement of a shaft, corresponding to the sine of the elevation angle a. The firborne DME unit B7 provides an output, either electrical or mechanical, corresponding to the distance D of the aircraft from the ground station. A function multiplier 83 is connected to the sine function generator 8l and the DME unit 6'! and provides an output corresponding to the product D sine a of the two inputs. This is representative of the height of the aircraft above the horizontal plane extending through the ground station.

As is known to those skilled in the art, the sine function generator 8l may include a constant voltage source and a voltage divider tapered to provide a sinusoidal resistance vs. rotation characteristic. The function multiplier mav be rrerely r variable gain amplifier whose gain is controlled by the output of the function generator 8l. The height indicator in this case would be a voltmeter 35, suitably calibrated in terms of height.

An alternative arrangement is shown in Fig. 12, wherein the elevation servomotor 57 operates a shaft 81 connected to a pinion 89, and the DME servomotor ll is coupled through a shaft el to a lead screw 93. The lead screw is supported in suitable bearings on an arm which is pivoted near one of its ends at the point lill. The free end of the arm 95 carries a toothed sector 99 in engagement with the pinion t9. A slidable pivot assembly lfll is carried radially along the arm S5 by the lead screw s3. The pivot assembly l 5l is also slidable in a slotted horizontal member 103 which is provided at one of its ends with a rack H35 suitably restrained by bearings, not shown, for vertical rrotion only. The rack W5 engages a pinion lill which is connected through a shaft lIlQ to a dial and pointer arrangement lll. The device lll is calibrated in terms of height.

In the operation of the system of Fig. 12. the arm 95, and with it the lead screw 23 is ricaintained at the angle a with respect to the horizontal bv the elevation servo 5l. The radial distance of the pivot point ll from the point el is controlled by the DME servo 'll in accordance with the slant distance D of the aircraft from the ground station. The vertical height of the pivot point lill above the point 97 is thus proportional to D sine a and to the height H of the aircraft fbove the reference plane. The displacement of the rack |05 from its reference position is equal to the height of point llil to accesso 'point standA thus corresponds tothe height 'ci theaircraftivhich is indicated'by the device Hi.

Although the' vertical plane coversge of the bcamproduced by thel antenna `arrangement oi Fig. 1 may be mede iarge'enoueii to include any practical 'range 'of glide path angles, .it may be desirable to provide-a' down indicationon aircraft which is' Weil Vabovethe useful'upper limit 'of' the beam. This' indication neednotjbe quantitative; for instance, 'an oif=sca1`e deectioi'i of the pointer of the meter 4? would be sufncient;

The described function may be obtained-byproviding an 'auxiliary 'radiator HS' behind tire*- lens 26 and below theA directive antenna 3, and suppiying it with `energy from tlietraiismitter Vinod-u`- lated directly at 360 cycles', with attivelylafige hired phr'se difference with 'respect to the"refer` enceY phase generator V2c. f The antenna H5 be simply a hori'ontal'dipole, fed from trans-` critter l through a modulator Hi. "The device f-r H? 'maybe Yamechanical modulator-sueas a paddle ,vh'e'el 'or vertr bie 'capacitor driven by the shaft I8, or `a small ampliner modulated `bv 'the output of the reference phase geneartor 29.

Since many changes could be 'made in the above constructionand many apparently Widely differs ent embodiments of this iirfentioncould` be' made without deprrting frornthe scope thereof, it is intendedthat vall matter contained in theY above description or shown in theaccomanying drawe ings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A radio glide path system including a ground str tion comprising a radio transmitter, antenna means for radiating directively the output of said transmitter in a beam of substantial vertical thickness, means for cyclically modifying the phase front of said beam in the vertical plane whereby the intensity of said berm along .any elevation line therein varies, the phase of said radiationbeing a function of saidv elevation, means for producing a reference phase vsignal and modulating said transm' ter thereby; et least one aircraft station comprising a receiver responsive to said transmissions from said groundl station to reproduce said reference phase signaland to produce a variable phase signrl corresponding to said variations in said phase front of said beam, and phase comparison means connected to said receiver' and responsive to` 'said Yl'cst mentioned signals to indicate the phase relationship therebetween, in terms of thek eleva-tion angle of said aircraft station.

2. The invention as set forth in claim l, including at said ground station further radictor means energized by said transmitter 'to provide a second beam of constant phase front directed above said first beam, and means for modulating the energy applied to said further radiator with* a signal like said reference phase signal but displaced in phase with respect thereto.

3. A radio height iinder system for aircraft comprising means including a transmitter at a reference location for establishing a directive radiation field which varies in intensity in characteristic phase at different angles of elevation with respect to said reference point, a reference phase generator connected to said transmitter to modulate said field with a reference phase signal, an aircraft station including receiver means for reproducing said reference phase modulation and producing a signal corresponding to said elevation-characteristic intensity variations, and phase comparator means responsive to said two l' (il last mentioned signal-s to provide an output representative of the elevation angle of said aircraft stati-on with respect to said reference point; radio distance measuring equipment for providing on said aircraft an output representative of the disur tance of said craft from said reference point, and means responsive to said' two last mentioned outputs to provide an indication of the height of said craft above 'the plane of said reference point.

radioA height finder system for aircraft comprising -ineans including a transmitter at a reference location foi" establishing a directive 11a-'- diation field which varies in intensity in charactei'istic plia-seh'ac'coruing to the angle of elevae tion With-respect-to said reference point, a refer-- ence 'phase generator connected to said transmiti ter toV modulate said neld with a reference phase Signal, and an aircraitsta tion including means ree Sponsive to said radi-ation nerd to provide an out-Y putrepresentative of the elevation angle of said aircraitV station with respect to said reference point; radio distance Vmeasuring equipment for providing on said aircraft an output representaerence point,V and-means responsive to saidtwo last Amentioned Aoutputs t'ol provide an indication ci' the height vof sai-d ncraft above the plane of said reference point.

5. In acraoio glide path'syste'm of the described Vproviding elevation angiev information in terms" of the phase diner-ence vbetweentvv-o `sigiiais produced 'on an aircraft', means for controilingtne 'curvatmfe or' the giidepath to be oiiotvevd' by saidaircraft incid-ding radio distance measuring equiprnent'foi" producing on said craft a signal representant/*dof the distance thereof ironia rererencepoina"and phase 'shifter "meansl res onsive to'saio iastinentioned signal to vary the phase 'f one' 'f`said`fst mentioned Signals' respect" t theoti" cor'dihg 'to said 'desired curvature. I

"6. Ina raoioglide pathH ysti 'of thedescribed ypiill'u'ding phase `compris`on means providg"ivatln ziiigll'ifiiidftlolf in ieSpOhS to the' phase differencebetvveen tvvosignalsprduced on an'aircrart, coursesoft'ening means incli'i'ding "radio distance measuring eduipment foi" producing on said craft a signal representative of the' distance thereof from a reference point, andineans responsive to said last mentioned signal to vary the response sensitivity of said phase comparison means'.

system i'or flying aircraft along a selected glide path, including a station at a reference location comprising a transmitter, means for radi ating the output of said transmitter in a beam extending throughout a sector which includes said glide path, means periodically varying the phase front of said beam to modulate each eieva tional element thereof in a phase corresponding to the respective elevation /of said element, and means for generating and transmitting a refer ence phase signal; a mobile station on said aircraft comprising means responsive to the trans mission from said reference station to reproduce said reference phase signal and means responsive to the modulation of said beam to produce a further signal Whose phase corresponds to the elevation angle of said craft, means for adjusting the phase of said reproduced reference phase signal to correspond with the elevation angie of said selected glide path, and indicator means responsive to said adjusted reference signal and to 1i said further signal to indicate deviation of said crai't from said glide path.

8. A system for flying aircraft along a selected glide path, including a station at a reference location comprising a transmitter, a directive antenna for radiating the output of said transmitter in a beam extending throughout a vertical sector which includes said glide path, means for periodically varying the phase front of said beam to vary the intensity of each elevational element thereof in a phase corresponding to the respective elevation of said element, means for generating a reference phase signal and modulating said transmitter with said signal; a mobile station on said aircraft comprising means responsive to the transmission lroin said reerence station to re produce said reference phase signal and to produce a further signal whose phase corresponds to said variation in intensity of said beam according to the elevation angle of said craft, means for adjusting the phase of said reproduced reierence phase signal, and means responsive to said adiusted reference signal and to said further signai to indicate elevation or' said cralt.

9. A system for flying aircraft along a selected glide path, including a station at a reference location comprising means lor radiating a beam extending continuously throughout a sector which includes said glide path, means for modulating said beam to vary each elevational element thereor' in a pnase corresponding to the respective elevation of said element, means for generating a reference phase signal and transmitting said signal to the aircraft; a mobile station on said aircraft comprising means I'or reproducing said reference phase signal and means for producing a further signal whose phase corresponds to said modulation of said beam according to the elevation angle of said craft, and indicator means responsive to said reference signal and to said further signal to indicate the elevation of said craft.

10. A radio glide path system for aircraft, including a transmitter, directive antenna means connected to said transmitter and arranged to provide a radio beam of substantial vertical thickness including the range of elevation angles through which the system is to be used, means for modulating said beam in a phase which is different at each elevation, the phase of said modulation being characteristic of the elevation angle, and means for producing and transmitting a constant phase reference signal.

11. An antenna for radio glide path systems and the like, including a plane-polarized radiator element, a spaced pair ol' conductive plates in planes perpendicular to trie direction oi polarization or said element and on opposite sides of said element, meaiis ior rotating said plates about the axis or polarization oi said element, and piiase front mouir'yiiig means comprising pairs oi conductive rods supported between said plates and movable therewith around said radiator element, each pair being collinear and parallel to said axis.

12. Apparatus of the class described, including an aircralt station comprising a radio receiver adapted to receive radio signals modulated in a priase which varies according to elevation and also modulated in a reierence phase, said receiver including means Ior demoduiating salu radio signals and separately reproducing the modulation signals, means for adiusting the phase oi' one or said modulation signals, arid phase comparison -means responsive to said adJusted modulation signal and to the other of said modulation signals to indicate the phase dinerence between them.

is. The invention set forth in claim 11, including an electromagnetic horn enclosing a sector around said axis, said phase Iront modifying means being angularly spaced around said axis at intervals approximately equal to the angular width of said sector.

JOSEPH LYMAN. i GEORGE B. LITCHFORD.

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